mirror of https://github.com/mkerrisk/man-pages
283 lines
7.1 KiB
Groff
283 lines
7.1 KiB
Groff
.\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
|
|
.\"
|
|
.\" %%%LICENSE_START(VERBATIM)
|
|
.\" Permission is granted to make and distribute verbatim copies of this
|
|
.\" manual provided the copyright notice and this permission notice are
|
|
.\" preserved on all copies.
|
|
.\"
|
|
.\" Permission is granted to copy and distribute modified versions of this
|
|
.\" manual under the conditions for verbatim copying, provided that the
|
|
.\" entire resulting derived work is distributed under the terms of a
|
|
.\" permission notice identical to this one.
|
|
.\"
|
|
.\" Since the Linux kernel and libraries are constantly changing, this
|
|
.\" manual page may be incorrect or out-of-date. The author(s) assume no
|
|
.\" responsibility for errors or omissions, or for damages resulting from
|
|
.\" the use of the information contained herein. The author(s) may not
|
|
.\" have taken the same level of care in the production of this manual,
|
|
.\" which is licensed free of charge, as they might when working
|
|
.\" professionally.
|
|
.\"
|
|
.\" Formatted or processed versions of this manual, if unaccompanied by
|
|
.\" the source, must acknowledge the copyright and authors of this work.
|
|
.\" %%%LICENSE_END
|
|
.\"
|
|
.\" Modified Sat Jul 24 19:00:59 1993 by Rik Faith (faith@cs.unc.edu)
|
|
.\" Clarification concerning realloc, iwj10@cus.cam.ac.uk (Ian Jackson), 950701
|
|
.\" Documented MALLOC_CHECK_, Wolfram Gloger (wmglo@dent.med.uni-muenchen.de)
|
|
.\" 2007-09-15 mtk: added notes on malloc()'s use of sbrk() and mmap().
|
|
.\"
|
|
.TH MALLOC 3 2013-12-12 "GNU" "Linux Programmer's Manual"
|
|
.SH NAME
|
|
malloc, free, calloc, realloc \- allocate and free dynamic memory
|
|
.SH SYNOPSIS
|
|
.nf
|
|
.B #include <stdlib.h>
|
|
.sp
|
|
.BI "void *malloc(size_t " "size" );
|
|
.BI "void free(void " "*ptr" );
|
|
.BI "void *calloc(size_t " "nmemb" ", size_t " "size" );
|
|
.BI "void *realloc(void " "*ptr" ", size_t " "size" );
|
|
.fi
|
|
.SH DESCRIPTION
|
|
.PP
|
|
The
|
|
.BR malloc ()
|
|
function allocates
|
|
.I size
|
|
bytes and returns a pointer to the allocated memory.
|
|
.IR "The memory is not initialized" .
|
|
If
|
|
.I size
|
|
is 0, then
|
|
.BR malloc ()
|
|
returns either NULL,
|
|
.\" glibc does this:
|
|
or a unique pointer value that can later be successfully passed to
|
|
.BR free ().
|
|
.PP
|
|
The
|
|
.BR free ()
|
|
function frees the memory space pointed to by
|
|
.IR ptr ,
|
|
which must have been returned by a previous call to
|
|
.BR malloc (),
|
|
.BR calloc ()
|
|
or
|
|
.BR realloc ().
|
|
Otherwise, or if
|
|
.I free(ptr)
|
|
has already been called before, undefined behavior occurs.
|
|
If
|
|
.I ptr
|
|
is NULL, no operation is performed.
|
|
.PP
|
|
The
|
|
.BR calloc ()
|
|
function allocates memory for an array of
|
|
.I nmemb
|
|
elements of
|
|
.I size
|
|
bytes each and returns a pointer to the allocated memory.
|
|
The memory is set to zero.
|
|
If
|
|
.I nmemb
|
|
or
|
|
.I size
|
|
is 0, then
|
|
.BR calloc ()
|
|
returns either NULL,
|
|
.\" glibc does this:
|
|
or a unique pointer value that can later be successfully passed to
|
|
.BR free ().
|
|
.PP
|
|
The
|
|
.BR realloc ()
|
|
function changes the size of the memory block pointed to by
|
|
.I ptr
|
|
to
|
|
.I size
|
|
bytes.
|
|
The contents will be unchanged in the range from the start of the region
|
|
up to the minimum of the old and new sizes.
|
|
If the new size is larger than the old size, the added memory will
|
|
.I not
|
|
be initialized.
|
|
If
|
|
.I ptr
|
|
is NULL, then the call is equivalent to
|
|
.IR malloc(size) ,
|
|
for all values of
|
|
.IR size ;
|
|
if
|
|
.I size
|
|
is equal to zero,
|
|
and
|
|
.I ptr
|
|
is not NULL, then the call is equivalent to
|
|
.IR free(ptr) .
|
|
Unless
|
|
.I ptr
|
|
is NULL, it must have been returned by an earlier call to
|
|
.BR malloc (),
|
|
.BR calloc ()
|
|
or
|
|
.BR realloc ().
|
|
If the area pointed to was moved, a
|
|
.I free(ptr)
|
|
is done.
|
|
.SH RETURN VALUE
|
|
The
|
|
.BR malloc ()
|
|
and
|
|
.BR calloc ()
|
|
functions return a pointer to the allocated memory,
|
|
which is suitably aligned for any built-in type.
|
|
On error, these functions return NULL.
|
|
NULL may also be returned by a successful call to
|
|
.BR malloc ()
|
|
with a
|
|
.I size
|
|
of zero,
|
|
or by a successful call to
|
|
.BR calloc ()
|
|
with
|
|
.I nmemb
|
|
or
|
|
.I size
|
|
equal to zero.
|
|
.PP
|
|
The
|
|
.BR free ()
|
|
function returns no value.
|
|
.PP
|
|
The
|
|
.BR realloc ()
|
|
function returns a pointer to the newly allocated memory, which is suitably
|
|
aligned for any built-in type and may be different from
|
|
.IR ptr ,
|
|
or NULL if the request fails.
|
|
If
|
|
.I size
|
|
was equal to 0, either NULL or a pointer suitable to be passed to
|
|
.BR free ()
|
|
is returned.
|
|
If
|
|
.BR realloc ()
|
|
fails the original block is left untouched; it is not freed or moved.
|
|
.SH CONFORMING TO
|
|
C89, C99.
|
|
.SH NOTES
|
|
By default, Linux follows an optimistic memory allocation strategy.
|
|
This means that when
|
|
.BR malloc ()
|
|
returns non-NULL there is no guarantee that the memory really
|
|
is available.
|
|
In case it turns out that the system is out of memory,
|
|
one or more processes will be killed by the OOM killer.
|
|
For more information, see the description of
|
|
.IR /proc/sys/vm/overcommit_memory
|
|
and
|
|
.IR /proc/sys/vm/oom_adj
|
|
in
|
|
.BR proc (5),
|
|
and the Linux kernel source file
|
|
.IR Documentation/vm/overcommit-accounting .
|
|
|
|
Normally,
|
|
.BR malloc ()
|
|
allocates memory from the heap, and adjusts the size of the heap
|
|
as required, using
|
|
.BR sbrk (2).
|
|
When allocating blocks of memory larger than
|
|
.B MMAP_THRESHOLD
|
|
bytes, the glibc
|
|
.BR malloc ()
|
|
implementation allocates the memory as a private anonymous mapping using
|
|
.BR mmap (2).
|
|
.B MMAP_THRESHOLD
|
|
is 128 kB by default, but is adjustable using
|
|
.BR mallopt (3).
|
|
Allocations performed using
|
|
.BR mmap (2)
|
|
are unaffected by the
|
|
.B RLIMIT_DATA
|
|
resource limit (see
|
|
.BR getrlimit (2)).
|
|
|
|
To avoid corruption in multithreaded applications,
|
|
mutexes are used internally to protect the memory-management
|
|
data structures employed by these functions.
|
|
In a multithreaded application in which threads simultaneously
|
|
allocate and free memory,
|
|
there could be contention for these mutexes.
|
|
To scalably handle memory allocation in multithreaded applications,
|
|
glibc creates additional
|
|
.IR "memory allocation arenas"
|
|
if mutex contention is detected.
|
|
Each arena is a large region of memory that is internally allocated
|
|
by the system
|
|
(using
|
|
.BR brk (2)
|
|
or
|
|
.BR mmap (2)),
|
|
and managed with its own mutexes.
|
|
|
|
The UNIX 98 standard requires
|
|
.BR malloc (),
|
|
.BR calloc (),
|
|
and
|
|
.BR realloc ()
|
|
to set
|
|
.I errno
|
|
to
|
|
.B ENOMEM
|
|
upon failure.
|
|
Glibc assumes that this is done
|
|
(and the glibc versions of these routines do this); if you
|
|
use a private malloc implementation that does not set
|
|
.IR errno ,
|
|
then certain library routines may fail without having
|
|
a reason in
|
|
.IR errno .
|
|
.LP
|
|
Crashes in
|
|
.BR malloc (),
|
|
.BR calloc (),
|
|
.BR realloc (),
|
|
or
|
|
.BR free ()
|
|
are almost always related to heap corruption, such as overflowing
|
|
an allocated chunk or freeing the same pointer twice.
|
|
.PP
|
|
Recent versions of Linux libc (later than 5.4.23) and glibc (2.x)
|
|
include a
|
|
.BR malloc ()
|
|
implementation which is tunable via environment variables.
|
|
For details, see
|
|
.BR mallopt (3).
|
|
.SH SEE ALSO
|
|
.\" http://g.oswego.edu/dl/html/malloc.html
|
|
.\" A Memory Allocator - by Doug Lea
|
|
.\"
|
|
.\" http://www.bozemanpass.com/info/linux/malloc/Linux_Heap_Contention.html
|
|
.\" Linux Heap, Contention in free() - David Boreham
|
|
.\"
|
|
.\" http://www.citi.umich.edu/projects/linux-scalability/reports/malloc.html
|
|
.\" malloc() Performance in a Multithreaded Linux Environment -
|
|
.\" Check Lever, David Boreham
|
|
.\"
|
|
.ad l
|
|
.nh
|
|
.BR brk (2),
|
|
.BR mmap (2),
|
|
.BR alloca (3),
|
|
.BR malloc_get_state (3),
|
|
.BR malloc_info (3),
|
|
.BR malloc_trim (3),
|
|
.BR malloc_usable_size (3),
|
|
.BR mallopt (3),
|
|
.BR mcheck (3),
|
|
.BR mtrace (3),
|
|
.BR posix_memalign (3)
|